You feel the scratch at the back of your throat, and somebody hands you a bag of zinc lozenges. A common strength is 13.3 mg of zinc each.
In one trial the federal fact sheet describes, 50 adults took a lozenge of exactly that strength every two to three waking hours. Run that arithmetic for a day and you land somewhere between 60 and 100 mg of zinc. The federal upper limit for an adult is 40 mg a day, and it counts the zinc in your dinner along with the zinc in the lozenge.
That gap is not an accident, and nobody is hiding it. It is what the trials did. The studies reporting a benefit used doses above the published ceiling on purpose, and the federal office that publishes the ceiling reports those studies a few paragraphs away on the same page.
The two zinc numbers, and what each one covers
The NIH Office of Dietary Supplements publishes two zinc figures that answer two different questions.
Start with the Recommended Dietary Allowance, the target figure; article 322 sets out how the whole family of reference values is built. For adults 19 and older, the Office of Dietary Supplements lists 11 mg a day for men and 8 mg a day for women, rising to 11 mg in pregnancy and 12 mg in lactation. The values come from the Food and Nutrition Board at the National Academies, and the fact sheet, updated 6 January 2026, prints the table without attaching a year to the figures.
The other number, the Tolerable Upper Intake Level or UL, is 40 mg a day. That figure holds from age 19 onward and does not change in pregnancy or lactation.
Scope matters more than people realize. The Office of Dietary Supplements writes that “The FNB has established ULs for zinc from food and supplements for healthy individuals,” so 40 mg covers your total from both, not just what comes out of a bottle. ODS carves out one group. Anyone taking zinc as a medical treatment, under a physician’s care, sits outside the 40 mg figure entirely. Article 323 covers how upper limits are built.
A third figure sits on labels rather than in the DRI tables. FDA fixes it in 21 CFR 101.9, its food labeling regulation, and the zinc entry reads 11 mg for anyone aged 4 and up.
Shortfall is not rare either. The same page reports that 15 percent of U.S. adults 19 and older have zinc intakes below the Estimated Average Requirement, the intake meeting the needs of half a group. For girls aged 14 to 18 the figure is 20.9 percent. For boys the same age, 1 percent.
Run the lozenge arithmetic
On the common cold, the Office of Dietary Supplements reports mixed trial results. In that 50-adult trial, lozenges started within 24 hours of onset cut colds by three days against placebo. In another, 13.3 mg zinc gluconate lozenges shortened illness by one day while 5.0 and 11.5 mg zinc acetate lozenges did nothing. The page also reports an earlier review finding that doses over 75 mg a day reduced cold duration while lower doses did not, and a 2021 review in which most trials used total daily doses of 45 mg to 300 mg for up to two weeks.
Set those against the 40 mg upper limit. Seventy-five milligrams is not a little over the line. It is 1.875 upper limits in a day, and the 300 mg top of the trial range is seven and a half of them stacked. Because the limit counts food and supplements together, whatever you ate sits on top of that.
The usual move is to resolve that tension, and neither source resolves it for you.
The honest version is that the two numbers describe different exposures. The limit is built around intake carried on indefinitely, and the harm it was set to prevent, copper depletion, takes weeks to appear. Lozenge protocols run for days. But no federal agency publishes a short-course figure, so a reader who wants a number for that situation does not have one.
That is the state of the record. A regimen with trial evidence behind it exceeds the ceiling by design, and the ceiling’s publisher reports both facts without reconciling them.
Two 2024 reviews read the same trials and disagree
The literature started with a coincidence. A three-year-old girl with leukemia let a zinc tablet dissolve in her mouth instead of swallowing it, and her cold symptoms cleared within hours. Her father, George Eby, ran the first randomized trial. Dissolving rather than swallowing seemed to be the point, which suggested a local effect in the mouth and throat rather than a nutritional one.
The Cochrane review, led by Daryl Nault and published in May 2024, pooled 34 studies and 8,526 participants. For treatment, it found colds may be shorter with zinc: a mean difference of 2.37 days, with a 95 percent confidence interval running from 4.21 days shorter to 0.53 days shorter. A confidence interval is the range the true effect probably falls in, and this one is wide. The reviewers rated the evidence low-certainty, meaning further research could easily change it. Heterogeneity, the degree to which pooled studies disagree with each other, was 97 percent. For prevention, the risk ratio was 0.93 with an interval crossing 1.00, also low-certainty. Non-serious side effects were probably more common with zinc treatment.
Five months later, Harri Hemilä of the University of Helsinki and Elizabeth Chalker of the Australian National University published a critique in Frontiers in Medicine arguing that the pooling itself was invalid.
Their case has three parts. The Cochrane treatment analysis combined lozenge trials with trials of zinc put into the nose, which are not the same treatment: inside that single analysis, one lozenge trial gave 190 mg of zinc a day and one nasal trial gave 0.046 mg a day, roughly a 4,300-fold difference. Measuring in days is the wrong scale, they argue, because colds run from one day to three weeks and a one-day cold cannot be shortened by 2.37 days. And lozenge composition was ignored: some lozenges in the trials contained citric acid, tartaric acid or mannitol-sorbitol, which bind zinc so it is not released freely.
Their reanalysis, restricted to lozenges and run on a relative scale, found that zinc lozenges shortened colds in adults by 37 percent, confidence interval 27 to 46 percent. They found no evidence that lozenges shorten colds in children, and reported that the one pediatric trial differed from the adult trials at P = 0.0001.
Both papers are peer reviewed. The argument is about statistical method and what belongs in the pool, not about the underlying data. The takeaway is narrower than a verdict: if there is a benefit, the evidence points to it being specific to a form, a composition and an age group.
The earlier Cochrane review was withdrawn
An earlier Cochrane review of zinc for the common cold, by Meenu Singh and Rashmi Ranjan Das, was withdrawn in April 2015, with the withdrawal notice updated in September 2016. The trigger was comments filed through the Cochrane Library by Harri Hemilä in February 2015, identifying multiple errors in the review and alleging plagiarism of text and data from Hemilä’s own 2011 systematic review.
The outcome was mixed. Text replication was found, limited to short phrases, acknowledged by the authors, and judged minor enough to fix with a correction. On the data allegation, the Editor in Chief considered the authors’ explanation not conclusive, and the review stayed withdrawn. The JAMA clinical synopsis of the same work was retracted, with the notice published on 27 December 2016.
Two things follow, neither a scandal. The process ran in public and left a documented record, which is more than most sources of health claims offer. And the person who filed the 2015 complaint is one of the two authors of the 2024 critique. Small field, long argument, recurring names.
The nasal version and the loss of smell
On 16 June 2009, the FDA warned consumers to stop using and discard three intranasal zinc cold products from one manufacturer: a nasal gel, a swab product, and a kids-size swab that had been discontinued. The agency said it had received more than 130 reports of anosmia, the medical term for losing the sense of smell, and that it was concerned the loss may be permanent. It sent a warning letter to the manufacturer, stating the products could not be marketed without FDA approval.
The clinical picture was described in 2006 by Thomas Alexander and Terence Davidson at the University of California, San Diego, in 17 patients at a nasal dysfunction clinic. Each reported sniffing deeply while applying the gel, then immediate burning lasting minutes to hours, then loss of smell within 48 hours. Seven never developed any upper respiratory infection, which matters: a cold alone can take your smell away, and these patients did not have one.
The other side exists, and the record is genuinely split. Burton Slotnick and colleagues, in The Laryngoscope in 2007, gave mice intranasal zinc gluconate at 2, 8 or 50 microliters, roughly 4, 15 and 94 times the equivalent recommended human dose. Only the 50-microliter dose did much, and those mice were hyposmic, meaning their sense of smell was reduced, rather than anosmic. Input to the olfactory bulb nearly fully recovered within three weeks. The authors wrote that their outcomes “fail to support the claims from recent clinical case reports” about zinc gluconate nasal spray and anosmia.
A third study points back the other way. Jae Lim and colleagues at the University of Washington, in PLoS One in 2009, tested that zinc gluconate nasal product and several other intranasal agents on mouse and human nasal tissue. The zinc product was the only substance showing significant cytotoxicity, meaning cell-killing damage, in both. Treated mice could not detect novel odorants and had not recovered after two months.
So: adverse event reports and a case series in humans, tissue damage in one animal study, near-complete recovery in another. The FDA acted on reports rather than on an experiment, which is how safety signals in marketed products usually work.
The inspection file almost nobody reads
FDA’s published inspection record for the manufacturer holds detail that rarely reaches consumer coverage. For March 2005, an FDA investigator sorted the company’s complaint forms by hand and counted 36 loss-of-smell complaints among 148 safety complaints for that single month; the company’s own exhibit for the same month came to 38. Complaint data compiled by the company put anosmia at roughly 3.6 reports per 100,000 units sold for its two largest-selling nasal zinc products, peaking near 6.7 per 100,000 in 2004.
What the main federal fact sheet leaves out
The Office of Dietary Supplements zinc fact sheet, updated 6 January 2026, mentions zinc nasal sprays three times: once listing them among zinc-containing products, once noting that over-the-counter nasal sprays sold for colds could cause zinc toxicity if overused, and once noting that some cold trials used nasal sprays or gels. On a targeted read of the full page, it carries nothing about anosmia, nothing identifying loss of smell as a risk of nasal zinc, no mention of any FDA action, and no appearance of the year 2009.
A different NIH component does cover it. The National Center for Complementary and Integrative Health states on its common cold page that intranasal zinc has been linked to severe and permanent loss of smell, and that the FDA warned consumers about intranasal zinc products sold as cold remedies.
The item is on the federal record. It is just not on the page a careful person would check first, which is a reminder that one authoritative source is a starting point, not a complete answer.
Zinc and copper, the slow version of too much
The documented harm from too much zinc over time is not dramatic. It is copper.
The mechanism fits in a sentence: zinc and copper compete for the same absorption pathway in the gut, so a large, sustained zinc intake crowds copper out. That comes from a 2021 case report by Ahsan Wahab and colleagues in the Journal of Community Hospital Internal Medicine Perspectives.
The intakes are lower than people expect, and the two federal pages give two figures. The zinc fact sheet states that doses of 50 mg of zinc or more over a period of weeks can inhibit copper absorption. The copper fact sheet uses a different marker: reductions in erythrocyte copper-zinc superoxide dismutase, a blood measure of copper status, have been reported at zinc intakes of roughly 60 mg a day for up to 10 weeks. The copper page says directly that this is one reason the Food and Nutrition Board set the zinc upper limit at 40 mg a day.
What low copper does is where the danger sits. The 2021 case report describes hypocupremia, meaning low blood copper, causing anemia, low white cells, low platelets, or all three at once, with or without neurological problems. Their patient was an 81-year-old woman taking zinc for macular degeneration who arrived with low white cells, low platelets and moderate anemia. The authors note a lag of about 12 months between symptom onset and diagnosis, during which patients often undergo invasive testing such as bone marrow biopsy.
Nothing in that picture says “supplement,” which is exactly why it gets missed. Food is not the route either: the Office of Dietary Supplements notes that zinc from food is rarely as high as 50 mg.
The denture cream nobody counts
The strangest zinc exposure on the fact sheet is not a supplement at all. It reports that some denture adhesive creams contain 17 to 34 mg of zinc per gram of product, and describes the exposure of concern as people using at least one tube totaling 70 g or more of cream each week.
Put those two published figures together. Seventy grams a week is 10 grams a day, and at 17 to 34 mg a gram that is roughly 170 to 340 mg of zinc a day going into someone’s mouth, four to eight times the 40 mg limit. That multiplication is arithmetic off the fact sheet’s own numbers rather than a finding it states, and how much gets absorbed is not quantified there.
The broader point survives the caveat. The limit counts food and supplements. It has no line for adhesive.
The fact sheet also notes that very high supplemental doses, which it puts at 142 mg a day, might interfere with magnesium absorption. Its list of interacting medications, naming quinolone and tetracycline antibiotics, penicillamine and thiazide diuretics, is explicitly a few examples. Article 677 handles interactions.
Five questions that decide what a zinc study is about
A reading tool, not a protocol. These are what change the answer.
- Which route? Lozenge, capsule, syrup and nasal spray are different treatments with different evidence and different hazards. The 2024 dispute turned partly on a review that pooled two of them.
- How much elemental zinc, over how many days? A lozenge used eight times across five days and a tablet used daily for a year are different exposures even when the milligram figure matches.
- What else is in the lozenge? Hemilä and Chalker argue citric acid, tartaric acid and mannitol-sorbitol bind zinc so it never gets free in the mouth and throat.
- Adults or children? The 37 percent figure is an adult figure. Its authors say there is no evidence for children.
- What else in the day contains zinc? The limit is a total, and totals include the multivitamin and the cereal.
When this becomes a conversation with a clinician
Specific situations where the decision belongs to a professional, and only you have what they need.
Unexplained anemia, low white cells or low platelets in someone taking high-dose zinc for months. The copper connection is easy to miss, and the case literature describes a diagnostic lag of about a year.
Numbness, tingling or unsteadiness on the feet in a long-term zinc user. Copper deficiency can show up in the nervous system with or without blood changes.
Burning in the nose after using a nasal product, followed by loss of smell. FDA’s 2009 statement said the agency was concerned the loss may be permanent, and the clinic case series describes burning first, smell loss within 48 hours. The same goes for smell that has not returned after a cold has otherwise cleared.
Anyone taking zinc for a medical reason. The 40 mg limit does not apply to them, and what figure does is a clinical judgment rather than a published table.
In all of these, the most useful thing you can do is name the product. The 2021 case report describes patients cycling through costly workups because nobody connected the blood results to a bottle on the counter.
The three things the box does not print
With zinc, the number on the front of the box is the least informative number in the room.
What decides whether an exposure is the one that was studied, the one that is safe, or neither, is three things the box does not print: which form it is, what your total across everything comes to in a day, and how long you keep it up. The two 2024 reviews disagree about whether lozenges shorten colds. Neither disagrees about that.
Sources
- NIH Office of Dietary Supplements. “Zinc: Fact Sheet for Health Professionals.” ods.od.nih.gov
- NIH Office of Dietary Supplements. “Copper: Fact Sheet for Health Professionals.” ods.od.nih.gov
- National Center for Complementary and Integrative Health. “The Common Cold and Complementary Health Approaches: What the Science Says.” nccih.nih.gov
- U.S. Food and Drug Administration. “Warnings on Three Zicam Intranasal Zinc Products (archived FDA Consumer Update, posted June 16, 2009; copy filed in Matrixx Initiatives v. Siracusano and hosted by the U.S. Court of Appeals for the Ninth Circuit).” cdn.ca9.uscourts.gov
- U.S. Food and Drug Administration. “Matrixx Initiatives, Inc. (Zicam), Scottsdale, AZ: Form 483 and Establishment Inspection Report.” fda.gov
- U.S. Food and Drug Administration (eCFR). “21 CFR 101.9, Nutrition labeling of food (Reference Daily Intakes).” ecfr.gov
- PubMed. “Nault D, Machingo TA, Shipper AG, et al. "Zinc for prevention and treatment of the common cold." Cochrane Database of Systematic Reviews, 2024. DOI: 10.1002/14651858.CD014914.pub2.” doi.org
- PubMed. “Hemila H, Chalker E. "Shortcomings in the Cochrane review on zinc for the common cold (2024)." Frontiers in Medicine, 2024. DOI: 10.3389/fmed.2024.1470004.” doi.org
- PubMed. “Singh M, Das RR. "WITHDRAWN: Zinc for the common cold." Cochrane Database of Systematic Reviews, 2015 (withdrawal notice). DOI: 10.1002/14651858.CD001364.pub5.” doi.org
- PubMed. “Das RR, Singh M. "Notice of Retraction: Das RR, Singh M. Oral Zinc for the Common Cold. JAMA. 2014;311(14):1440-1441." JAMA, 2016. DOI: 10.1001/jama.2016.18134.” doi.org
- PubMed. “Slotnick B, Sanguino A, Husband S, et al. "Olfaction and olfactory epithelium in mice treated with zinc gluconate." Laryngoscope, 2007. DOI: 10.1097/MLG.0b013e318033006b.” doi.org
- PubMed. “Lim JH, Davis GE, Wang Z, et al. "Zicam-induced damage to mouse and human nasal tissue." PLoS One, 2009. DOI: 10.1371/journal.pone.0007647.” doi.org
- PubMed. “Alexander TH, Davidson TM. "Intranasal zinc and anosmia: the zinc-induced anosmia syndrome." Laryngoscope, 2006. DOI: 10.1097/01.mlg.0000191549.17796.13.” doi.org
- PubMed. “Wahab A, Mushtaq K, Khan A, et al. "Zinc-induced hypocupremia and pancytopenia, from zinc supplementation to its toxicity, a case report." Journal of Community Hospital Internal Medicine Perspectives, 2021. DOI: 10.1080/20009666.2021.1983319.” doi.org